SYNTHESIS AND CHARACTERIZATION OF NICKEL MANGANITE THIN FILMS FOR APPLICATION IN UNCOOLED MICROBOLOMETERS

Open Access
Author:
Schulze, Heidi M
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 17, 2008
Committee Members:
  • Susan E Trolier Mckinstry, Thesis Advisor
Keywords:
  • CHEMICAL SOLUTION DEPOSITION
  • MICROBOLOMETER
  • THIN FILM
  • SPINEL
  • INFRARED DETECTION
Abstract:
The nickel manganite thermistor thin films were studied as potential candidates for application in uncooled microbolometers, where low temperature processing, high TCR coupled with low noise, and property stability of the material are required. The phase formation within the nickel manganite materials system was studied with respect to annealing conditions (air, argon, oxygen, and engineered low pO2 from residual organics) through the synthesis of chemical solution deposited (acetate and acetylacetonate-based solutions) thin films over a wide range of compositions (Mn/Mn+Ni = 0.14 to 0.9) and temperatures (300 to 1030°C). Thin films were structurally characterized using X-ray diffraction and transmission electron microscopy (TEM) analysis and electrically characterized through determination of the temperature coefficient of resistance (TCR), resistivity, and activation energy. It has been shown that reducing atmospheres favor the spinel phase at low temperatures, while oxygen is required to crystallize thermodynamically stable phases with higher oxidation states on the Mn. The annealing atmospheres can be controlled either globally (via the furnace ambient) or locally (by leaving residual organics following the pyrolysis step) to control phase formation. Low temperature synthesis of the thermistor films is important to enable compatibility of the material with CMOS integration. Single-phase metastable cubic spinel films (Mn/Mn+Ni = 0.5 to 0.8) annealed in argon at 400°C for 5 hours exhibit TCR values ranging from -3.8 to -4.5 %/K and resistivity values on the order of 10,000 Ωcm. Metastable spinel films show excellent stability when heat-treated at 400°C for long times. Cubic spinel was also formed above 730°C with comparable properties to the bulk, including TCR values between -3.3 and -4.5 %/K and resistivities around 1000 Ωcm. The small lattice parameters of the films indicate a cation-deficient spinel structure. Due to the low processing temperatures possible, dense films could be prepared within the spinel phase field. Thus, decomposition into the NiO phase and a Mn – rich spinel, which is problematic in bulk ceramics, will not occur in such spinel thin films. For films prepared outside of the single-phase field, phase separation cannot always be detected using X-ray diffraction (due to the possibility of coherent straining or to the very small crystallite size). In such cases, transmission electron microscopy was useful in identifying decomposition. Single-phase spinel films (TCR = -3.6 to -4.1 %/K) were compared to single-phase bixbyite films (TCR = -3.1 to -3.3 %/K) synthesized between 630°C and 930°C. The bixbyite phase exhibits lower TCR and lower resistivity, 400-1600 Ωcm, compared to spinel, 3500-21,000 Ωcm. Composite films (achieved by controlling the pyrolysis to create a low local pO2) during annealing exhibit intermediate values (TCR = -3.0 to -3.8 %/K and resistivity = 470-6600 Ωcm).